The flagellum of many bacteria appears to be made up of a single protein known as flagellin. The serotyping schemes of E. coli and Salmonella enterica are based on highly variable antigenic surface structures which include the lipopolysaccharide which carries the O antigen and flagellin which is now known to be the carrier of the classical H antigen. In many strains of S. enterica there are two loci (fliC and fljB) which encode flagellin, and a regulatory system which allows one only to be expressed at any time; and which also provides for expression to rapidly alternate between the two forms first identified as two phases (H1 and H2) for the H antigen of most strains. In E. coli there are 54 forms of H antigen recognised and until recently they were all thought to be encoded at the fliC locus, as has been shown for E. coli K-12. However in the 1980s Ratiner [Ratiner Y A “Phase variation of the H antigen in Escherichia coli strain Bi327-41, the standard strain for Escherichia coli flagellin antigen H3” FEMS Microbiol. Lett 15 (1982) 33–36; Ratiner Y A “Presence of two structural genes determining antigenically different phase-specific flagellins in some Escherichia coli strains” FEMS Microbiol. Lett. 19 (1983) 37–41; Ratiner Y A “Two genetic arrangements determining flagellin antigen specificities in two diphasic Escherichia coli strains” FEMS Microbiol. Lett. 29 (1985) 317–323; Ratiner Y A “Different alleles of the flagellin gene hagB in Escherichia coli standard H test strains” FEMS Microbiol Lett. 48 (1987) 97–104] showed that in some cases there are two loci and that expression can alternate. The matter was further complicated by a recent paper by Ratiner [Ratiner Y A (1998) “New flagellin-specifying genes in some Escherichia coli strains” J. Bacteriol. 180 979–984] showing three loci (flk, fll and flm) for flagellin in addition to fliC although the fljB locus has not been found in E. coli. However E. coli strains are normally identified by the combination of one O antigen and one H antigen [and K antigen when present as a capsule (K) antigen], with no problems reported for the vast majority of cases with alternate phases, while S. enterica strains are normally identified by the combination of O, H1 and H2 antigens. It is still not clear how widespread in E. coli H antigens determined by flagellin genes other than fliC are.
Typing is typically carried out using specific antisera. The incidence of pathogenic E. coli in association with human and animal disease supports the need for suitable and rapid typing techniques.